Three groups of Merino ewes (a total of 150) kept in yards were studied for a period of 1 year. Vasectomized rams were with one group and occurrence of oestrus was noted. The presence and number of corpora lutea were observed by laparotomy at 6-week intervals. The annual rhythms in these characteristics were determined.
A second group of ewes received progesterone treatments at intervals of 6 weeks. Half the ewes were with vasectomized rams continuously but rams were with the others for only 1 week after the final progesterone injection. The period from the final injection to the onset of oestrus was determined. Introduction of rams increased the number of ewes showing oestrus at the beginning (late summer) and end (late winter) of the breeding season. The period to onset of oestrus was related to dose and frequency of progesterone injections, the time of the year, and introduction of rams. Periods to onset of oestrus became progressively longer after mid-winter (June) and a proportion of progesterone-treated ewes did not show oestrus in the 1 week period after cessation of progesterone injections in July, at a time when all untreated ewes were experiencing oestrous cycles. The implications of the seasonal change in the effects of progesterone on the ewes are discussed in relation to the sexual season.
A third group of ovariectomized ewes received progesterone followed by oestrogen, also at intervals of 6 weeks. The number of ewes showing oestrus during the period January to June was dependent on the dose of progesterone independently of the effect of dose of oestrogen. The relationship did not hold at other times. In general, there was considerable heterogeneity in mean responses and slopes of the dose-response lines.
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